[PATCH 06/11] KVM: x86/mmu: Revert "Revert "KVM: MMU: zap pages in batch""

Sean Christopherson
2019-09-13T02:46:51Z

Now that the fast invalidate mechanism has been reintroduced, restore
the performance tweaks for fast invalidation that existed prior to its
removal.

Paraphrashing the original changelog:

Zap at least 10 shadow pages before releasing mmu_lock to reduce the
overhead associated with re-acquiring the lock.

Note: "10" is an arbitrary number, speculated to be high enough so
that a vCPU isn't stuck zapping obsolete pages for an extended period,
but small enough so that other vCPUs aren't starved waiting for
mmu_lock.

This reverts commit 43d2b14b105fb00b8864c7b0ee7043cc1cc4a969.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---
arch/x86/kvm/mmu.c | 35 +++++++++--------------------------
1 file changed, 9 insertions(+), 26 deletions(-)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 0bf20afc3e73..827414b12dbd 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5670,12 +5670,12 @@ int kvm_mmu_create(struct kvm_vcpu *vcpu)
return alloc_mmu_pages(vcpu);
}

-
+#define BATCH_ZAP_PAGES 10
static void kvm_zap_obsolete_pages(struct kvm *kvm)
{
struct kvm_mmu_page *sp, *node;
LIST_HEAD(invalid_list);
- int ign;
+ int nr_zapped, batch = 0;

restart:
list_for_each_entry_safe_reverse(sp, node,
@@ -5688,28 +5688,6 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
break;

/*
- * Do not repeatedly zap a root page to avoid unnecessary
- * KVM_REQ_MMU_RELOAD, otherwise we may not be able to
- * progress:
- * vcpu 0 vcpu 1
- * call vcpu_enter_guest():
- * 1): handle KVM_REQ_MMU_RELOAD
- * and require mmu-lock to
- * load mmu
- * repeat:
- * 1): zap root page and
- * send KVM_REQ_MMU_RELOAD
- *
- * 2): if (cond_resched_lock(mmu-lock))
- *
- * 2): hold mmu-lock and load mmu
- *
- * 3): see KVM_REQ_MMU_RELOAD bit
- * on vcpu->requests is set
- * then return 1 to call
- * vcpu_enter_guest() again.
- * goto repeat;
- *
* Since we are reversely walking the list and the invalid
* list will be moved to the head, skip the invalid page
* can help us to avoid the infinity list walking.
@@ -5717,14 +5695,19 @@ static void kvm_zap_obsolete_pages(struct kvm *kvm)
if (sp->role.invalid)
continue;

- if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
+ if (batch >= BATCH_ZAP_PAGES &&
+ (need_resched() || spin_needbreak(&kvm->mmu_lock))) {
+ batch = 0;
kvm_mmu_commit_zap_page(kvm, &invalid_list);
cond_resched_lock(&kvm->mmu_lock);
goto restart;
}

- if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))
+ if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list,
+ &nr_zapped)) {
+ batch += nr_zapped;
goto restart;
+ }
}

kvm_mmu_commit_zap_page(kvm, &invalid_list);
--
2.22.0